This invention generally relates to electrochemical methods for removing at least one metallic coating from a substrate. In some of the more specific embodiments, the invention is directed to methods for selectively stripping aluminum-containing coatings from metal substrates.
A variety of coatings are used to provide oxidation resistance and thermal barrier properties to metal articles, such as turbine engine components. Current coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings. State-of-the-art diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide.
Overlay coatings typically have the composition MCrAl(X), where M is an element from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.
When articles such as gas turbines are serviced, the protective coatings usually must be removed to permit inspection and possible repair of the underlying substrate, followed by re-coating. Removal of the coatings is typically carried out by immersing the component in a stripping solution. A variety of stripping techniques are currently available for removing different types of coatings from metal substrates. The techniques usually must exhibit a considerable amount of selectivity. In other words, they must remove only intended materials, while generally preserving the article""s desired structures.
In the case of metallic coatings like those based on aluminum, one example of a particular stripping technique is chemical etching. In such a process, the article is submerged in an aqueous chemical etchant. The metallic coating on the article surface is then dissolved as a result of reaction with the etchant.
While many stripping techniques are very useful for a variety of applications, they may not always include the features needed in specialized situations. As an example, many forms of chemical etching are generally nonselective, and can result in undesirable loss of the substrate material. This material loss can lead to changes in critical dimensions, e.g., turbine airfoil wall thickness or cooling hole diameter. The material loss can also lead to structural degradation of the substrate alloy, e.g., by way of intergranular attack. Moreover, chemical etching can result in the stripping of coatings from internal passages in the article, which is often undesirable.
Masking techniques can be used to protect portions of a component""s structure from the effects of stripping solutions. For example, masking is often used to protect the internal cooling passages and holes in turbine engine components. However, masking and the subsequent removal of the masks can be time- and labor-consuming, detracting from the efficiency of a repair process.
Electrochemical stripping processes overcome some of the disadvantages inherent in conventional techniques such as chemical etching. For example, a patent application filed on Oct. 15, 1999 for Bin Wei et al, Ser. No. 09/420,059, describes a very useful electrochemical stripping process. In general, the process selectively removes metallic coatings from the external sections of a metallic article, such as a turbine component. The process employs an electrolytic solution based on various compounds, such as organic and inorganic salt/solvent systems. Examples of electrolytic systems are ammonium chloride/ethylene glycol, and aqueous sodium chloride. An advantage of this type of process is that coatings on internal passageways of the component generally remain unaffected by the action of the stripping agentxe2x80x94even when they have not been masked.
The invention of patent application Ser. No. 09/420,059 possesses novel features which are very useful for some applications. However, additional improvements are desirable in other situations. For example, ammonium chloride-type electrolytes can sometimes damage the base metal of an article. Moreover, sodium chloride-based electrolytes may not provide the xe2x80x9cthrowing powerxe2x80x9d sometimes required to strip articles which have complex shapes. Furthermore, the use of sodium chloride and some of the other inorganic salts can require specialized equipment, such as electrodes with highly conformal geometries. This requirement can add to the overall cost of the stripping process.
Moreover, some of the electrochemical stripping processes do not provide a wide enough xe2x80x9cprocess windowxe2x80x9d for efficient commercial operation. For example, the time period between complete stripping of the coating and the occurrence of significant damage to the substrate may be too short.
The need for a significant process window can be especially important in the case of aluminum-based diffusion coatings for metal substrates. Such coatings usually include two regions or xe2x80x9csublayersxe2x80x9d: an additive sublayer which lies on top of the base metal, and a diffusion sublayer below the additive sublayer, which is incorporated into the upper region of the base metal. Repeated stripping and re-applications of these coatings necessitate repeated removal of the diffusion sublayer, which can undesirably decrease the thickness of the substrate, e.g., a turbine airfoil. Thus, it is often desirable to remove only the additive sublayer when repairing the component, without significantly removing the diffusion sublayer. In this situation, stripping processes which do not slow down or cease after the additive sublayer has been removed are often impractical in an industrial setting.
It should thus be apparent that new stripping processes for removing coatings from substrates would be welcome in the art. The processes should include the advantageous features of known stripping techniques, while avoiding at least some of their deficiencies. For example, the new processes should be capable of removing substantially all of a given coating material, while not substantially attacking the substrate. The processes should also minimize or completely eliminate the need for masking. They should also preserve the structural and dimensional integrity of the parent alloy, as well as internal passages and cooling holes which may be located within an article (e.g., a turbine component).
Ideally, the new stripping processes would also include additional processing advantages. For example, they should not result in the formation of an unacceptable amount of hazardous fumes in the workplace, or effluent which cannot easily be treated. Moreover, the processes should include process windows (e.g., between the time when coating layers are removed but other layers and the substrate are preserved) which provide flexibility and efficiency in a large-scale treatment facility.
A primary embodiment of this invention is directed to an electrochemical stripping method for selectively removing at least one coating from the surface of a substrate. The substrate is often a superalloy material, e.g., a turbine engine component. The method includes the step of immersing the substrate in an aqueous composition through which electrical current flows. The composition comprises an acid having the formula HxAF6, or precursors to said acid. xe2x80x9cAxe2x80x9d is Si, Ge, Ti, Zr, Al, or Ga; and x is 1-6. Various coatings can be removed, such as diffusion coatings (e.g., aluminide-based) or overlay coatings of the MCrAl(X)-type. As used herein, the term xe2x80x9cremoval of a coatingxe2x80x9d is meant to refer to the severe degradation of the coating, leaving (at most) only a coating residue which weakly adheres to the underlying surface. The residue is easily removed by a subsequent, conventional technique such as xe2x80x9cde-smuttingxe2x80x9d, as discussed below.
The method of this invention can be used in a xe2x80x9cfull stripxe2x80x9d operation, where an entire coating is removed, or in a xe2x80x9cpartial stripxe2x80x9d operation. In the latter case, only one portion of a coating is removed. For example, the additive sublayer of a diffusion coating can be removed effectively and completely, while retaining the diffusion sublayer, as further described below. The electrical cell potential within the aqueous composition is adjusted to maximize the efficiency and selectivity of the process.
Another embodiment of the invention is directed to a method for replacing a worn or damaged protective coating applied over a substrate. The coating to be replaced is electrochemically removed by the process described below, i.e., using the HxAF6, electrolyte. A new coating can then be applied by any appropriate technique, e.g., aluminiding processes, high velocity oxy-fuel (HVOF), plasma spray, physical vapor deposition, and the like. As also described below, this embodiment is especially useful in the case of repairs for diffusion-aluminide coatings applied to substrates having rigorous dimensional requirements.
Still another embodiment of this invention relates to an apparatus for the electrochemical removal of at least one coating from a substrate. Features of the apparatus are described in detail below. In brief, it includes: (a) an electrolyte which comprises an acid having the formula HxAF6, as described herein; (b) an electrical current source capable of being connected to the coated substrate (i.e., the anode) and an electrode (i.e., the cathode); and (c) at least one electrode from which the electrical current source can apply electrical current through the electrolyte to the coated substrate.
The electrolyte for the apparatus is usually incorporated into a stripping bath in which the coated substrate can be immersed.
Further details regarding the various features of this invention are found in the remainder of the specification.